In recent years, there is a multi hop networks in which wireless devices that measure data, such as a water level, a temperature, or the like, are arranged in multiple locations and the data measured by each of the wireless devices is sent and received. Hereinafter, the wireless devices are simply referred to as nodes. In the multi hop network, if each of the nodes is connected by using a wired connection, a cable laying work is complicated. Accordingly, in the multi hop network, a study has been conducted on a technology that wirelessly connects each of the nodes and then sends and receives data by using a predetermined communication channel selected from the radio frequency bandwidth.
In the multi hop network in which each of the nodes are wirelessly connected, there is a problem of radio frequency interference in which, when nodes that are arranged close to each other simultaneously send and receive data by using a communication channel with the same radio frequency bandwidth, the radio waves from the nodes interfere with each other. As the technology that solves this radio frequency interference problem, there is a known Collision Avoidance mechanism, such as Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism or Request To Send/Clear To Send (RTS/CTS) mechanism. With the CSMA/CA mechanism, before each node transmits a radio wave, each of the nodes checks a sending status of a radio wave of the other node that is located in the same communication channel. With the RTS/CTS, before each node transmits a radio wave, each of the nodes sends a query, to a master station that is referred to as an access point, about the availability of the transmission of a radio wave in the same communication channel.
However, with the CSMA/CA mechanism or the RTS/CTS mechanism, each of the nodes avoids the collision between radio waves by using the same communication channel and thus each of the nodes is not able to send and receive data until the other node located in the vicinity of the node stops the transmission of the radio wave. Consequently, the throughput may possibly be reduced.
In contrast, there is a proposed technology that arbitrates a collision between radio waves by using multiple communication channels that can send and receive data. With this technology, a master station that performs overall control of multiple nodes searches the multiple communication channels that can send and receive data for a free channel that is not used by the other node and then selects the searched free channel as a communication channel that sends and receives the data. The master station broadcasts a beacon signal that includes information on the selected communication channel to the nodes controlled by the master station. The nodes received the beacon signal starts to send and receive the data by using the free channel that is indicated by the beacon signal.    Patent Document 1: Japanese Laid-open Patent Publication No. 2002-158667
However, with the conventional technology described above, there is a problem in that the nodes arranged outside the reach range of the beacon signal are not able to select an appropriate communication channel. If the nodes are not able to select an appropriate communication channel, the radio waves from the nodes may possibly interfere with each other and thus the throughput may possibly be reduced.
In the following, the problem of the conventional technology will be described. FIG. 24 is a schematic diagram illustrating a problem of a conventional technology. In the example illustrated in FIG. 24, nodes 10a to 10h are wirelessly connected. In this example, it is assumed that the node 10a is a master station, it is assumed that the node 10a selects, as the communication channel that is used to send and receive data, a free channel that is searched from multiple communication channels with the radio frequency bandwidth. The node 10a that is the master station broadcasts a beacon signal that includes therein information on the selected free channel. A node 10b is arranged in a reach range B of the beacon signal sent from the node 10a. Consequently, after the node 10b receives the beacon signal, the node 10b can send and receive data to and from the node 10a by using the free channel that is indicated by the beacon signal. In contrast, the nodes 10c to 10h are arranged outside the reach range B of the beacon signal. Consequently, the nodes 10c to 10h are not able to recognize the free channel indicated by the beacon signal and thus is not able to select an appropriate communication channel. Thus, the nodes 10c to 10h may possibly disorderly select the same communication channel. In such a case, the radio waves from the nodes interfere with each other and thus the throughput is reduced.